Process for partially electroplating flat silver

ABSTRACT

A process for partially silver electroplating the metal surface of a flat silver part wherein part of the surface is covered prior to silvering with a plating-resistant dipping enamel. A separation surface is provided between the exposed surface of the part to be covered with the dipping enamel and the exposed surface of the lengthwise adjacent part to be silvered. The separation surface consists of a material which is more difficult to wet with the dipping enamel than the metal of the surface to be covered. Preferably, the separation surface is the peripheral surface of a spacer lamination composed of polytetrafluoroethylene.

The invention relates to a process for partially silver electroplatingflat silver parts such as the handle of a table knife.

It is difficult to achieve a precise, optically flawless boundary whenpartially silver electroplating flat silver parts. As used herein, theterm "flat silver" refers to forks, spoons and knives.

In the manufacture of table knives with silvered handles, for example,once the blade has been set into the handle, the transition site must beground flush, that is, the so-called blade crop must be ground back asfar as the contour of the blade. The procedure generally used involvessilvering the handle, inserting the blade, and grinding flush thetransition site, thus providing for an optimal boundary of the silveredsurface. The flush-grinding step, however, is very costly since it canonly be done by hand and requires a highly skilled workman. Further,since the silver layer of the handle must not be damaged, a slight step,wherein food residues may catch, remains at the blade crop even underthe most favorable circumstances.

Moreover, complete silvering of an assembled and ground knife, whereinthe silver coating must be mechanically removed from the blade, has alsobeen found to be unsuitable since this method is only effective withsoft silver that will not adhere to the steel of the blade. When hardsilver is employed, the silver coating, owing to the underlayer ofnickel contained in the hard silver, adheres so firmly to the steelblade that it cannot be removed.

There have thus been many attempts to join two components of a flatsilver part, such as the handle and the blade, prior to silvering, togrind the transition site until no step is left, and to avoid contact ofthe blade with the silver during the subsequent silvering process.

German patent application No. P 28 29 776.9 describes a process in whichthe blade is electrically insulated with respect to the handle and makescontact with the handle only through the cathode in the electroplatingprocess. Certain difficulties inherent in this process have continued toprevent this method from becoming practical.

The application of well-known electroplating technology, relating toenameling, to a process for partially silvering flat silver, such as thehandles of table knives, has previously failed. Specifically, when theblade is dipped into the liquid enamel, a precise boundary of theenameling cannot be obtained at the gap between the handle and the cropbecause (1) it is extremely difficult in mass production to dip partsdown to within a fraction of a millimeter of a specific mark; and (2) itis nearly impossible to maintain the constant physical parametersrequired to wet uniformly the blade surface with the dipping enamel.

For example, when a blade is dipped into a bath, the enamel initiallypulls back slightly from the area of contact. If the blade remainsmotionless, however, the enamel rises toward the blade and forms acharacteristic wall angle corresponding to the interface tension betweenthe blade material and the dip enamel, which interface tension dependson the surface tensions of the materials involved.

It has been found experimentally that the wall angle forming between theblade material and the enamel cannot be predicted with sufficientaccuracy. Specifically, because of the variable surface tension of thematerial to be wetted, the wall angle varies across the surface of asingle part, i.e., for an absolute constant level of the dip enamelbath, the enamel boundary may be more or less serrated and may noteverywhere reach the gap between blade crop and handle.

Thus, any impurity on the blade surface, whether sweat traces fromfinger prints or fatty residues from abrasives, can so alter the localsurface tension that the same enamel will wet one blade but not the nextand will in fact wet differently across the same blade. Accordingly,since the wall angle and hence the dip depth of a part to be coatedcannot be predetermined with the required accuracy, the enamel coatingtherefore (1) may not extend to the mark, i.e., the gap between theblade and the handle; (2) it may extend beyond the mark; or (3) bothdefects may simultaneously be present.

It is the object of the present invention to create a process for thepartial silvering of flat silver, wherein a precise boundary between thesilvering and the surface not to be silvered will be achieved andwherein the silvering, especially during the manufacture of tableknives, can be carried out on the assembled and ground workpiece.

The object of the present invention is achieved by providing aseparation surface between the surface of the component to be coveredwith dipping enamel and the surface of the component to be silvered.This separation surface is wetted poorly or not at all by the dipenamel. The unpredictable wetting behavior of the dip enamel is thusbuffered, so that variations in both interface tensions and, to someextent, dipping depths can be neutralized.

Thus, the dipping apparatus no longer needs such precise adjustment asheretofore. Moreover, operating personnel who dip manually no longerneed control their motions so sharply.

When manufacturing table knives and other flat silver assembled fromseveral pieces wherein the silvering must terminate precisely at thetransition site from one piece to the next, it is particularlyadvantageous that the separation surface be the peripheral surface of aspacer lamination inserted between the parts to be joined, such asbetween the crop of the blade and the handle.

The separation surface preferably consists of polytetrafluoroethylene.Surprisingly, it was found that when using this material, a separationsurface thickness of at least 0.1 mm, preferably 0.2 mm, suffices toretain the desired properties.

The concept of the invention is described below in further detail inrelation to the drawings.

FIG. 1 shows a knife being dipped into a dip enamel.

FIG. 2 is an enlarged representation of the termination of the dipenamel in the absence of a separation surface.

FIG. 3 is an enlarged representation of the termination of the dipenamel in the presence of a separation surface.

FIG. 1 shows a dipping system with a schematic clamping head 1 and a dipenamel bath 2 for a knife consisting of a handle 3 and a blade 4. Whenassembling the blade 4 and the handle 3, either by "soldering", bygluing with synthetics or by cementing, a somewhat protruding spacerlamination 5 is inserted between the blade 4 and the handle 3. The blade4 and the handle 3 are compressed hard while the bonding materialsharden; thus the spacing lamination 5 additionally becomes a seal forthe gap. After the bonding materials are hardened, the knife is groundflush, in which process the spacing lamination is leveled to the surfaceof the knife handle.

In the case of the knife without a spacer lamination, the meniscus shownin FIG. 2 with a wall angle θ less than 90° will be formed shortly afterthe blade 4 is dipped a little below the transition site of the handle 3into the dip enamel 2. As already mentioned, this angle depends on theratio of the surface tension of the dip enamel 2 to that of the blade 4.

A wall angle θ<90°, as is the case under discussion, means that thesurface tension of the material to be covered is large compared to thatof the dip enamel and that the blade surface will be wetted. Impuritiesmay so decrease the surface tension of the blade that the wall angle θbecomes larger than the 90° and the surface of the dipping bath will beas shown in curve a. The boundary of the enamel will then vary withinthe limits c in spite of the constant level A of the dipping bath.

As shown in FIG. 3, when a knife with a separation surface such asspacer lamination 5 is immersed into the dipping bath 2 as far as thetransition site from blade 4 to the handle 3, then the spacinglamination 5 prevents, even after dwell time, the formation of ameniscus, as θ exceeds 90° for non-wetting surfaces. The blade 4accordingly can be dipped as far as the spacing lamination 5 into thedipping bath without the dipping enamel rising in an unforeseeablemanner at the surface. This "braking effect" will tolerate a short-termdeeper dipping within the limits of levels B and C into the dippingenamel. Thus, the dipping apparatus no longer need be controlled asprecisely, i.e., if there is manual dipping, the operating personnelneed not control their motions so sharply.

In addition to possessing the property of low wetting, the material usedfor the spacer lamination 5, particularly when manufacturing tableknives, must be mechanically very stable, resistant to all sorts of foodand cleansing means, resistant to temperature effects up to about 200°C. and yet effective even when the lamination is quite thin.

A particularly preferred material is polytetrafluoroethylene. Inpractical tests with teflon laminated spacers, it was found that alamination of only 0.2 mm gives optimal results. Such a thin spacinglamination is hardly visible in the finished knife and thus in no wayaffects the overall aesthetic appearance.

Use of an interposed spacer lamination is not essential for achievingthe object of the present invention. The boundary area also may consistof a strip of material deposited on the surface or inserted into anexisting groove. Further, when a workpiece is exposed to reducedthermal, chemical or mechanical stresses, a more economical materialthan polytetrafluroethylene, but possessing the same wetting properties,may also be used.

What is claimed is:
 1. a process for partially silver electroplating aflat silver part, the part including two metal components to be joinedlengthwise, the first component to be covered with a plating-resistantdipping enamel and the second component to be electroplated with silver,the process comprising the steps of:(a) joining lengthwise the said twometal components; (b) providing a thin separation surface betweenexposed surfaces of said two components, said separation surfaceconsisting of a material which is more difficult to wet with the dippingenamel than the metal of the first component to be covered; (c) dippingsaid first component into said enamel until contact is achieved betweensaid enamel and said separation surface; and (d) silver electroplatingsaid second component.
 2. The process of claim 1, including, immediatelyprior to said dipping, the further step of grinding the transition siteof said two joined components.
 3. The process of claim 1, wherein saidtwo components are joined lengthwise to a spacer lamination, said spacerlamination separating exposed surfaces of said first and secondcomponents and further wherein the peripheral surface of said spacerlamination provides said separation surface.
 4. The process of claim 3wherein said flat silver part is a table knife, said first metalcomponent is a blade having a crop thereof, said second metal componentis a handle and said spacer lamination separates the crop of said bladefrom said handle.
 5. The process of claim 3, wherein said spacerlamination has a thickness of at least about 0.1 mm.
 6. The process ofclaim 5 wherein said spacer lamination has a thickness of about 0.2 mm.7. The process of claim 1, wherein said separation surface is made ofpolytetrafluoroethylene.
 8. In a process for partially silverelectroplating the metal surface of a flat silver part wherein, prior tosilvering a lengthwise adjacent part of the surface, a part of thesurface is covered with a plating-resistant dipping enamel, theimprovement comprising the step of:providing a separation surfacebetween an exposed surface of said part to be covered with the dippingenamel and an exposed surface of said lengthwise adjacent part to besilvered, said separation surface consisting of a material which is moredifficult to wet with the dipping enamel than the metal of the surfaceto be covered.